Integrated wavelength selector

ABSTRACT

Integrated wavelength selectors are described. The wavelength selector may include silicon nitride ring resonator disposed vertically between a heater and a temperature sensor. The temperature sensor may be formed of silicon in some embodiments. The wavelength selector may be coupled to the output port of a tunable laser, or may be disposed within a laser cavity.

BACKGROUND Field

The present application relates to optical wavelength lockers.

Related Art

Wavelength lockers are used to lock an optical signal to a particularwavelength. Some wavelength lockers employ ring resonators. Theresonance frequency of the ring resonator serves as a referencefrequency. The frequency of a signal of interest can be compared to thereference frequency provided by the ring resonator. The frequency of thesignal of interest can be controlled to match the resonance frequency ofthe ring resonator.

BRIEF SUMMARY

An integrated wavelength selector is provided, comprising: a substrate;an integrated heater on the substrate; an integrated silicon temperaturesensor on the substrate; and a ring waveguide of a first materialdisposed vertically between the integrated heater and the integratedsilicon temperature sensor, the first material exhibiting a lowertemperature coefficient of resonant frequency (TCF) than silicon.

According to an aspect of the present application, a tunable wavelengthselector is provided, comprising: a micro-heater; a micro-temperaturesensor formed of silicon; and an optical filter of a first materialdisposed vertically between the micro-heater and the micro-temperaturesensor, the first material exhibiting a lower temperature coefficient ofresonant frequency (TCF) than silicon.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments of the application will be describedwith reference to the following figures. It should be appreciated thatthe figures are not necessarily drawn to scale. Items appearing inmultiple figures are indicated by the same reference number in all thefigures in which they appear.

FIG. 1A is a perspective view of an integrated wavelength selector,according to a non-limiting embodiment of the present application.

FIG. 1B is a side view of the integrated wavelength selector of FIG. 1A.

FIG. 2 is a side view of an integrated wavelength selector of the typeshown in FIGS. 1A and 1B.

FIG. 3 illustrates a top view of a portion of a temperature sensor asmay be implemented in a wavelength selector, according to a non-limitingembodiment of the present application.

FIG. 4 illustrates a plan view of a wavelength selector comprising aplurality of thermal isolation trenches, according to a non-limitingembodiment of the present application.

FIG. 5 illustrates an apparatus comprising a laser and a wavelengthlocker coupled to an output port of the laser, according to anon-limiting embodiment of the present application.

FIG. 6 illustrates an apparatus comprising a tunable laser and awavelength locker array coupled to an output of the tunable laser,according to a non-limiting embodiment of the present application.

FIG. 7 illustrates an apparatus comprising a laser having a laser cavityincluding a wavelength selector coupled to the optical path by a tapcoupler, according to a non-limiting embodiment of the presentapplication.

FIG. 8 illustrates an apparatus comprising a laser having a laser cavityincluding a wavelength selector array coupled to the optical path by atap coupler, according to a non-limiting embodiment of the presentapplication

FIG. 9 illustrates an apparatus comprising a laser having a laser cavityincluding a wavelength selector disposed in the optical path, accordingto a non-limiting embodiment of the present application.

FIG. 10 illustrates an optical system comprising a laser, an integratedwavelength selector, and control circuitry, on a substrate, according toa non-limiting embodiment of the present application.

DETAILED DESCRIPTION

Aspects of the present application provide an integrated semiconductorwavelength selector comprising an optical filter with an integratedheater and an integrated temperature sensor. The temperature sensor maybe formed of silicon. The optical filter may be formed of a materialhaving a lower temperature coefficient of resonant frequency (TCF) thansilicon. For example, the optical filter may comprise silicon nitride.The optical filter, temperature sensor, and heater may be disposedvertically in a stack, with the optical filter between the temperaturesensor and the heater. In some embodiments, the optical filter is a ringresonator.

In some embodiments, an integrated semiconductor wavelength selectorconfigured as a wavelength locker is provided. For example, thewavelength selector may be coupled to an output of a laser, allowing forthe laser frequency to be locked to a reference frequency presented bythe wavelength locker. The reference frequency may be adjusted bycontrolling a temperature of the wavelength locker. In some embodiments,an array of wavelength lockers with different reference frequencies maybe coupled to a laser output. A tunable laser may be provided bycoupling a laser output to the wavelength locker or to an array ofwavelength lockers.

In some embodiments, an integrated semiconductor wavelength selector maybe disposed in a laser cavity to control the frequency output by thelaser. In such situations, the wavelength selector may operate to selectthe laser output frequency. In some embodiments, the wavelength selectoris coupled to the optical path within the laser cavity by a tap coupler.In some embodiments, the wavelength selector is disposed in the opticalpath within the laser cavity.

In some embodiments, arrays of integrated wavelength selectors areprovided. The arrays may include two or more integrated wavelengthselectors of the types described herein. The arrays may be used toprovide frequency tuning of an optical signal.

The aspects and embodiments described above, as well as additionalaspects and embodiments, are described further below. These aspectsand/or embodiments may be used individually, all together, or in anycombination of two or more, as the application is not limited in thisrespect.

FIG. 1A is a perspective view of an integrated wavelength selector,according to a non-limiting embodiment of the present application. Theintegrated wavelength selector 100 comprises a ring resonator 102,temperature sensor 104, and heater 106. The ring resonator represents anon-limiting example of an optical filter. Examples of other opticalfilters that may be used in a wavelength selector of the types describedherein include disk resonators, race-track resonators, and Fabry-Perotinterferometers. Returning to FIG. 1A, the ring resonator may comprisean input waveguide 108 a, an output waveguide 108 b, and a ringwaveguide 110. In some embodiments, such as that shown in FIG. 1A, thering resonator may comprise first and second waveguides disposed onopposite sides of the ring waveguide. FIG. 1B is a side view of theintegrated wavelength selector 100 of FIG. 1A.

In some embodiments, an integrated wavelength selector comprises a ringresonator of a different material than a temperature sensor of theintegrated wavelength selector. For example, referring to FIGS. 1A and1B, the ring resonator 102 may be a different material than thetemperature sensor 104. The material of the ring resonator 102 may beselected to exhibit relatively little change in resonance frequency withchanges in temperature. The resonance frequency of the ring resonator,or an offset from the resonance frequency, may serve as a referencefrequency. For some applications, precise control of the resonancefrequency may be desired. For example, certain applications of tunablelasers may call for control of the laser frequency within +/−1 GHz. Useof a wavelength selector to tune the frequency of such tunable lasersmay therefore call for control of the resonance frequency of thewavelength selector within +/−1 GHz. The resonance frequency of asemiconductor ring resonator may vary with temperature. According tosome embodiments, a ring resonator of an integrated wavelength selectoris formed of a material that exhibits a relatively low variation inresonance frequency for a given change in temperature. For example, thering resonator 102 may comprise silicon nitride. Silicon nitride has alower TCF than, for example, silicon. However, the ring resonator 102may be formed of other materials in alternative embodiments. Forexample, the ring resonator may comprise siliconoxynitride. In someembodiments, the ring resonator comprises SiOx or SiON. The ringresonator may be formed of a material compatible with siliconmicrofabrication processing. In some embodiments, the temperature sensorof an integrated wavelength selector of the types described herein maybe formed of silicon. For example, the temperature sensor 104 may beformed of silicon to be compatible with silicon processing technology.Referring again to FIG. 1A, as a non-limiting example, the ringresonator 102 may be formed of silicon nitride and the temperaturesensor 104 may be formed of silicon. In some embodiments, then, the ringresonator may be formed of a material different than the temperaturesensor.

As described above, in some embodiments an offset from the resonancefrequency of the ring resonator may be used as a reference frequency.For example, an offset of 1 GHz, 2 GHz, or between 1 GHz and 10 GHz fromthe resonance frequency may be used as the reference signal. In someembodiments, the frequency at which there is equal transmission throughthe input waveguide 108 a and the output waveguide 108 b is used as thereference frequency. The various aspects described herein are notlimited to use of the resonance frequency as the reference frequency.

As has been described, the integrated temperature sensor 104 of FIG. 1Amay be formed of silicon. In some embodiments, such as that of FIGS. 1Aand 1B, the temperature sensor 104 may be a p-n diode, wherein the p andn regions are shaped as rings. For example, the temperature sensor 104may be a silicon p-n diode with an outer ring doped p-type or n-type,and an inner ring doped the opposite type of the outer ring. In someembodiments, the temperature sensor may be a resistive temperaturesensor. For example, the temperature sensor may be a ring-shapedresistor of doped silicon. Returning to FIG. 1A, the heater 106 may beformed of a resistive material. For example, the heater 106 may beformed of TiN, TaN, or polysilicon. As a non-limiting example, the ringresonator 102 is formed of silicon nitride, the temperature sensor 104is a silicon p-n diode, and the heater 106 is a polysilicon resistiveheater. Other combinations of materials are possible for the integratedwavelength selector 100.

Referring to FIGS. 1A and 1B, the ring resonator 102, temperature sensor104, and heater 106 are stacked vertically. For example, referring toFIG. 1B, the ring resonator 102 may be above the temperature sensor 104in the z-direction. The heater 106 may be above the ring resonator 102in the z-direction. In some embodiments, such as that shown, anintegrated wavelength selector comprises a vertical stack of an opticalfilter, a heater, and a temperature sensor, with the optical filterbeing disposed between the heater and the temperature sensor. Theoptical filter may be a ring resonator, as shown in FIGS. 1A and 1B.Returning to FIG. 1B, the distance between the heater and the ringresonator may be selected to ensure low or no overlap of the opticalsignal from the ring resonator in the heater, while also ensuring theheater is sufficiently close to the ring resonator to heat it. Asopposed to silicon waveguides, silicon nitride and silicon oxynitridewaveguides do not confine light as tightly. The optical signal in suchwaveguides may leak from the waveguides into surrounding structures.Leakage of light from the waveguides into the heater may be undesirable.Returning to FIG. 1B, the spacing between the heater 106 and thewaveguides of the ring resonator 102 may be selected to avoid lightleaking from the waveguides into the heater.

Referring still to FIGS. 1A and 1B, the illustrated components may haveany suitable dimensions. For example, the ring waveguide 110 may have adiameter selected to provide a desired resonance frequency. As shown inFIGS. 1A and 1B, the temperature sensor 104 may be narrower than thering waveguide 110 in the x-y plane. In some embodiments, the heater 106may be sized to substantially overlap with the ring waveguide 110. Insome embodiments, such as that shown, the heater 106 may be a ringsubstantially overlying the ring resonator. The components may beseparated by any suitable spacing. For example, the ring resonator 102may be separated from the temperature sensor 104 in the z-direction bybetween 1 and 20 microns, including any value within that range,although other distances are possible. The heater 106 may be spaced fromthe ring resonator 102 by between 1 and 20 microns, including any valuewithin that range, although other distances are possible. As anon-limiting example, the ring waveguide has an inner diameter between10 and 150 microns, the temperature sensor has an inner diameternarrower than ring waveguide by between 5% and 25%, and the heater hasan inner diameter larger than the ring waveguide by between 1% and 20%.In some embodiments, the ring waveguide 110, temperature sensor 104, andheater 106 may have dimensions on the order of microns. The heater 106is a micro-heater in some embodiments. The temperature sensor 104 is amicro-temperature sensor in some embodiments. The ring waveguide has alarger radius than the temperature sensor 104 in some embodiments.

In operation, an input optical signal is provided to the ring resonator102 on input waveguide 108 a. For example, the input optical signal mayrepresent a tapped signal from an optical waveguide. For example, it maybe desired to know or control the frequency of an optical signaltraveling through an optical waveguide, so the optical signal may betapped to provide a portion thereof to the integrated wavelengthselector 100. Returning to FIG. 1A, the optical signal may couple fromthe input waveguide 108 a to the ring waveguide 110, and from the ringwaveguide 110 to the output waveguide 108 b. The amount of lightremaining on the input waveguide 108 a and on the output waveguide 108 bmay be detected using photodetectors. The detected amount of light, orthe ratio of the detected amount of light on the input waveguide 108 ato the amount of light on the output waveguide 108 b, may be used todetermine the frequency of the optical signal. The temperature sensor104 may detect the temperature of the integrated wavelength selector100. Information about the detected temperature may be used to controlthe heater 106. The heater 106 may be controlled to provide a desiredamount of heat to control the temperature of the integrated wavelengthselector 100. The temperature of the integrated wavelength selector 100may be controlled to provide a desired resonance frequency of theoptical filter, such as the ring resonator 102. In some embodiments, thetemperature sensor 104 and the heater 106 are coupled in a feedbackloop.

FIG. 2 is a side view of an integrated wavelength selector 200 of thetype shown in FIGS. 1A and 1B. The integrated wavelength selector 200comprises ring resonator 202, a temperature sensor 204, and a heater206. The temperature sensor 204 may be a silicon temperature sensordisposed in a semiconductor substrate 208 The semiconductor substrate208 may be a silicon substrate. The ring resonator 202 may comprise amaterial having a lower TCF than that of silicon. For example, the ringresonator 202 may be formed of any of the materials described previouslyin connection with ring resonator 102. The heater 206 may be a resistiveheater. The heater 206 may be formed of any of the materials describedpreviously in connection with heater 106. The ring resonator 202 andheater 206 may be disposed within an insulating layer 210. Theinsulating layer 210 may be silicon dioxide, or any other suitableinsulating material. As a non-limiting example of an implementation ofthe integrated wavelength selector 200, the semiconductor substrate 208is a silicon substrate, the temperature sensor 204 is a silicontemperature sensor, the ring resonator 202 is a silicon nitride ringresonator, the heater 206 is a resistive heater, and the insulatinglayer 210 is silicon dioxide. Integrated wavelength selectors of thetype illustrated in FIG. 2 may be formed of alternative combinations ofmaterials than those listed. In some embodiments, the semiconductorsubstrate 208 may be replaced with a dielectric substrate, such assilicon dioxide, and the temperature sensor 204 may be disposed in asemiconductor layer on the dielectric substrate.

FIG. 3 illustrates a top view of a portion of a temperature sensor asmay be implemented in a wavelength selector of the types describedherein, according to a non-limiting embodiment of the presentapplication. As described above in connection with FIGS. 1A and 1B, insome embodiments an integrated temperature sensor of a wavelengthselector may be a pn junction silicon temperature sensor. FIG. 3illustrates one half of a pn junction silicon temperature sensor of thetype that may be employed in an integrated wavelength selector. Thetemperature sensor 300 comprises an outer ring portion 302 and an innerring portion 304. One of the outer ring portion 302 and the inner ringportion 304 may be doped p-type, and the other may be doped n-type. Forexample, the outer ring portion 302 may be a p-type ring and the innerring portion 304 may be an n-type ring. Returning to FIG. 3 , the outerring portion 302 include an electrical contact tab 303. The inner ringportion 304 includes an electrical contact tab 305. The electricalcontact tabs 303 and 305 may facilitate making electrical contact to thetemperature sensor. As described above, and as shown in FIGS. 1A and 1B,an integrated temperature sensor may be disposed underneath a ringresonator in a vertical stack. Therefore, making electrical contact tothe temperature sensor may involve providing an electrical path thatcircumvents the ring resonator. Referring to FIG. 1A, electrical contactto the temperature sensor 104 may be made using conductive vias thatpass through a center of the ring resonator 102 and the heater 106.Returning to FIG. 3 , the electrical contact tabs 303 and 305 extendtoward a center of the temperature sensor, allowing electrical contactto be made by vias which may pass through a center of a ring resonatoroverlying the temperature sensor 300. As an alternative to a p-n diodetemperature sensor, in some embodiments the temperature sensor may be aresistive temperature sensor. For example, the temperature sensor may beformed of doped silicon, acting as a resistive silicon temperaturesensor. In some embodiments, the temperature sensor may be a ring-shapedresistor of doped silicon.

According to an aspect of the present application, an integratedwavelength selector comprises thermal isolation structures. As describedpreviously herein, an integrated wavelength selector may include anintegrated heater to control the temperature of the optical filter, suchas a ring resonator. For example, the resonance frequency of the ringresonator may be controlled by controlling the temperature of the ringresonator. As has also been described previously herein, an integratedwavelength selector may include a temperature sensor configured to sensea temperature of the integrated wavelength selector. In particular, thetemperature sensor may be employed to sense a temperature of the opticalfilter, such as a ring resonator. In combination, the temperature sensorand heater may provide precise control of the temperature of the opticalfilter, and thus precise control of its resonance frequency. Suchfunctionality may be facilitated or enhanced by confining the heat fromthe heater around the optical filter. Such functionality may befacilitated by confining the heat around the temperature sensor, so thatthe temperature sensor accurately senses the temperature of the opticalfilter. Accordingly, aspects of the present application provide thermalisolation structures as part of an integrated wavelength selector. Thethermal isolation structure may be positioned to confine heat from aheater around an optical filter and temperature sensor of the integratedwavelength selector.

FIG. 4 illustrates a plan view of a wavelength selector comprising aplurality of thermal isolation trenches, according to a non-limitingembodiment of the present application. The integrated wavelengthselector 400 comprises a temperature sensor 404, heater 406, and ringresonator formed by input waveguide 408 a, output waveguide 408 b, andring waveguide 410. The integrated wavelength selector 400 furthercomprises thermal isolation trenches 412. In some embodiments, thethermal isolation trenches extend through the entire stack including thetemperature sensor 404, heater 405, and ring resonator, to providethermal isolation of those components and a substantially homogenoustemperature among those components.

As shown in the non-limiting example of FIG. 4 , the temperature sensor404 is a pn diode of the type illustrated previously in connection withFIG. 3 , wherein the junction is formed between inner and outer dopingrings. The heater 406 overlies the ring waveguide 410 and thetemperature sensor 404. The thermal isolation trenches 412 areconfigured to confine the heat from the heater 406 radially within theregion in which the ring waveguide 410 and the temperature sensor 404are disposed. The thermal isolation trenches 412 facilitate uniformheating and a reduced temperature gradient within the region comprisingthe ring resonator and the temperature sensor. The thermal isolationtrenches may comprise a thermally insulating material. For example, thethermal isolation trenches 412 may be filled with silicon oxide, as anon-limiting example. The number and sizing of the thermal isolationtrenches 412 may be selected to provide a desired degree of thermalconfinement of the heat from the heater within the region comprising thering waveguide 410 and the temperature sensor 404. In at least someembodiments, the thermal isolation trenches 412 extend vertically atleast from the temperature sensor 404 to the heater 406. In someembodiments the thermal isolation trenches 412 extend from a depthbeneath the temperature sensor 404 to a height above the heater 406. Ashas been described previously, in some embodiments the temperaturesensor may be a resistive temperature sensor.

Integrated wavelength selectors of the types described herein may beconfigured to operate as wavelength lockers or as intra-cavitywavelength selectors, according to various embodiments of the presentapplication. A wavelength selector may be configured as wavelengthlocker in some embodiments. For example, coupling the wavelengthselector to an output of a laser may facilitate locking the laserfrequency to a reference wavelength presented by the wavelength locker.The resonance frequency of the wavelength selector may be controlled bycontrolling the temperature of the wavelength selector, and may be usedto provide a tunable wavelength locker. The tunable wavelength lockermay be coupled to a laser output to provide a tunable laser. In someembodiments, coupling an array of wavelength selectors of differentresonance frequencies to an output of a laser may provide for a tunablelaser. The laser may be turned to have a frequency matching a desiredreference frequency from the array of wavelength selectors. In theseembodiments, the wavelength selector or array of wavelength selectorsmay allow for locking the laser frequency to a desired frequency. Insome embodiments, a wavelength selector or array of wavelength selectorsmay form part of a laser cavity. The laser may laze when the laserfrequency matches the resonance frequency of the wavelength selector, orone of the wavelength selectors of the array of wavelength selectors. Inthis manner, the wavelength selectors may operate to select thefrequency of a laser output signal.

FIG. 5 illustrates an apparatus comprising a tunable laser and awavelength locker coupled to an output port of the tunable laser,according to a non-limiting embodiment of the present application. Theapparatus 500 comprises a laser 502 and a wavelength locker 508. Thelaser 502 comprises a gain medium 504, mirror 506 a, and mirror 506 b.The laser has an output port 512 configured to provide an output lasersignal. The wavelength locker 508 may be any of the types of integratedwavelength selectors described herein. For example, the wavelengthlocker 508 may be the integrated wavelength selector 100 of FIG. 1A.Returning to FIG. 5 , the wavelength locker 508 is coupled to the outputport 512 of the laser 502 by a coupling waveguide 510. The couplingwaveguide 510 is a tap coupler in some embodiments.

In operation, the laser 502 outputs laser light from the output port512. A portion of the laser light from output port 512 couples to thecoupling waveguide 510. The laser light is provided as an input to thewavelength locker 508. For example, the wavelength locker 508 may be theintegrated wavelength selector 100 of FIG. 1A. The laser light may beprovided as an input on the input waveguide 108 a. Returning to FIG. 5 ,the wavelength locker 508 may provide an indication of a frequency ofthe laser light provided at the output port 512. The laser 502 may betuned until the wavelength locker 508 indicates that the laser light hasa frequency substantially matching the reference frequency presented bythe wavelength locker 508. In this manner, the frequency of the laserlight provided at the output port 512 may be locked to the referencefrequency presented by the wavelength locker 508. Moreover, thereference frequency presented by the wavelength locker 508 may beadjusted. For example, as described previously in the context of FIG.1A, the resonance frequency of an integrated wavelength selector may beadjusted by controlling the temperature of the integrated wavelengthselector. Returning to FIG. 5 , in some embodiments the wavelengthlocker comprises an optical filter having a resonance frequency, and theresonance frequency of the wavelength locker 508 may be adjusted bycontrolling the temperature of the wavelength locker. In this manner,the laser light at the output port 512 may be locked to a variablefrequency. The laser 502 may be a tunable laser.

FIG. 6 illustrates an apparatus comprising a wavelength locker array.The apparatus 600 comprises several components in common with theapparatus 500. The same reference numbers are used for those components,and they are not described in detail again here. The apparatus furthercomprises a wavelength locker array 602 comprising a plurality ofwavelength lockers 508. The wavelength lockers 508 of the wavelengthlocker array 602 may have respective resonance frequencies. For example,the wavelength lockers 508 may have ring resonators of differentresonance frequencies due to differences in size, or may be heated todifferent temperatures using the respective heater. The laser signalproduced at output port 512 may be coupled to the wavelength lockerarray 602 by the coupling waveguide 510. The wavelength lockers 508 ofthe wavelength locker array 602 may be connected in series or inparallel. In either configuration, they may each provide an indicationof whether the frequency of the laser signal at output port 512 matchestheir respective resonance frequency. The laser 502 may be tuned tomatch one of the reference frequencies of the wavelength locker array.In this manner, the frequency of the laser signal may be selectable.

As described previously, aspects of the present application provide alaser comprising a wavelength selector of the types described hereindisposed in the optical cavity of the laser. As described previously,integrated wavelength selectors having an optical filter formed of amaterial with a lower TCF than silicon may provide greater temperaturestability than would be provided by an optical filter made of silicon.In some embodiments, use of an optical filter formed of a material otherthan silicon, such as silicon nitride, may increase temperaturestability of the wavelength selector by a factor of five or more.Greater temperature stability of the optical filter's resonancefrequency may relax the temperature control needed to achieve aparticular resonance frequency. For example, use of a silicon nitridering resonator as part of a wavelength selector may relax thetemperature control to achieve a particular frequency of the wavelengthselector by a factor of five or more compared to use of a silicon ringresonator. In some embodiments, relaxing the temperature control of theoptical filter may facilitate including the optical filter directly in alaser cavity, where precise temperature control can be challenging. Insome embodiments, stoichiometric Si₃N₄ may be used as a ring resonatorof an integrated wavelength selector. Its thermooptical coefficient isabout 2.5e⁻⁵ at 1550 nm and room temperature. Therefore, the temperaturecontrol accuracy of Si₃N₄ waveguides can be relaxed to about 0.5C, whichis 5 times of that of a silicon-based optical filter.

FIG. 7 illustrates an apparatus comprising a laser having a laser cavityincluding a wavelength selector coupled to the optical path by a tapcoupler, according to a non-limiting embodiment of the presentapplication. The apparatus 700 comprises several components in commonwith the apparatus 500. The same reference numbers are used for thosecomponents, and they are not described in detail again here. Theapparatus comprises a laser 702 having a laser cavity including the gainmedium 504 and mirrors 506 a-506 b. A wavelength selector 704 is alsoincluded within the laser cavity. The wavelength selector 704 may be anyof the types of wavelength selectors described herein. For example, theintegrated wavelength selector 100 of FIG. 1A may be used as thewavelength selector 704. The wavelength selector 704 is coupled to theoptical path 705 of the laser 702 by the tap coupler 510. The wavelengthselector 704 presents a reference frequency to which the laser signal inthe optical path 705 can be compared. For example, the wavelengthselector 704 may operate in the manner described previously inconnection with wavelength selector 100. As a result, the laser signaloutput from the output port 512 may be the same reference as thereference frequency presented by the wavelength selector. In someembodiments, the wavelength selector 704 is tunable through temperaturecontrol, in the manner describe previously in connection with FIG. 1A.Therefore, in some embodiments, a tunable laser is provided comprising awavelength selector disposed within a laser optical cavity and coupledto the optical path of the laser cavity by a tap coupler.

FIG. 8 illustrates an apparatus comprising a laser having a laser cavityincluding a wavelength selector array coupled to the optical path by atap coupler, according to a non-limiting embodiment of the presentapplication. The apparatus 800 comprises several components in commonwith the apparatus 700. However, laser 802 of the apparatus 800comprises a wavelength selector array 804 comprising a plurality ofwavelength selectors 704 of the type described in connection with FIG. 7. The wavelength selectors 704 of the wavelength selector array 804 mayhave respective resonance frequencies. For example, the wavelengthselectors 704 may have ring resonators of different resonancefrequencies due to differences in size, or may be heated to differenttemperatures using the respective heater. The laser signal in theoptical path 705 may be locked to one of the reference frequenciespresented by the wavelength selector array. In this manner, a tunablelaser may be achieved.

FIG. 9 illustrates an apparatus comprising a laser having a laser cavityincluding a wavelength selector disposed in the optical path, accordingto a non-limiting embodiment of the present application. The apparatus900 comprises a laser 901 having several components in common with theapparatus 700. The wavelength selector 902 is disposed in the opticalpath of the laser cavity. Thus, the laser signal may transmit in bothdirections through the wavelength selector 902. Lasing may occur whenthe laser signal in the laser cavity substantially matches the referencefrequency presented by the wavelength selector 902. The wavelengthselector 902 may be any of the types of wavelength selectors describedherein. In some embodiments, the wavelength selector may be a wavelengthselector array presenting multiple reference frequencies. In someembodiments, then, a plurality of integrated heaters, integrated silicontemperature sensors, and ring resonators are disposed in the cavity ofthe tunable laser.

FIG. 10 illustrates an optical system comprising a laser, an integratedwavelength selector, and control circuitry, on a substrate, according toa non-limiting embodiment of the present application. The optical system1000 comprises the laser 502 and wavelength locker 508 coupled to theoutput port 512 by the coupling waveguide 510. The system 1000 furthercomprises control circuitry 1002. The control circuitry 1002 is coupledto the wavelength locker 508 and the laser 502. The control circuitry1002 may comprise optical and/or electrical circuitry. The controlcircuitry may receive signals from the wavelength locker 508 and sendcontrol signals to the laser 502 and/or the wavelength locker 508. Forexample, in the non-limiting embodiment shown, the wavelength locker 508may output an output signal, or multiple output signals tophotodetectors 1003. For example, the wavelength locker 508 may comprisea wavelength locker of the type shown in FIG. 1A and photodetectors maybe positioned at ends of input waveguide 108 a and output waveguide 108b to detect the amount of light in those waveguides. The photodetectors1003 may provide electrical signals S1 and S2 to the control circuitry1002 indicative of the amount of light detected by the photodetectors,which in turn may be indicative of an amount of light detected by thewavelength locker 508. Such signals may provide an indication of thefrequency of the laser light output at output port 512. Based on thereceived signals, the control circuitry may provide a control signal S3to the laser 502 to tune the frequency of the laser. The wavelengthlocker 508 and control circuitry 1002 may also exchange signals S4.Signals S4 may include an output of the temperature sensor of thewavelength locker, provided to the control circuitry 1002. Signals S4may include a heater control signal sent from the control circuitry 1002to the heater of the wavelength locker 508 to control an amount of heatproduced by the heater. It should be appreciated that a systemconfiguration including photodetectors and/or control circuitry likethat shown in FIG. 10 may be used in connection with any of theembodiments of wavelength selectors and lasers described herein.

In FIG. 10 the photodetectors 1003 are shown as separate from thewavelength locker 508 and control circuitry 1002. In some embodiments, aphotodetector may be considered part of the wavelength locker or part ofthe control circuitry.

Returning to FIG. 10 , the illustrated components may be disposed on asubstrate 1004. In some embodiments, the components are monolithicallyintegrated on the substrate. For example, in some embodiments thesubstrate 1004 may be a semiconductor substrate and the components maybe monolithically integrated on the semiconductor substrate. In someembodiments, the substrate 1004 is a silicon substrate. The use of anintegrated wavelength selector of the types described herein may allowfor all, or substantially all, of the components illustrated in FIG. 10to be monolithically formed on a single substrate. In some embodiments,the components of optical system 1000 may be combined onto a singlesubstrate 1004 but may not be monolithically formed on the substrate.For example, the substrate 1004 may be a printed circuit board (pcb),and the components of the optical system may be positioned on theprinted circuit board. Forming the optical system 1000 on a singlesubstrate may provide significant cost and space savings compared tooptical systems utilizing discrete components formed on multiplesubstrates. The manufacturing of such a system may be less complex thana system using discrete components and multiple substrates.

It should be appreciated from the foregoing that aspects of the presentapplication provide tunable lasers. Tunable lasers may have any of theconfigurations illustrated in FIGS. 5-10 .

The terms “approximately” and “about” may be used to mean within ±20% ofa target value in some embodiments, within ±10% of a target value insome embodiments, within ±5% of a target value in some embodiments, andyet within ±2% of a target value in some embodiments. The terms“approximately” and “about” may include the target value.

What is claimed is:
 1. An integrated wavelength selector, comprising: asubstrate; an integrated heater on the substrate; an integrated silicontemperature sensor on the substrate; and a ring waveguide of a firstmaterial disposed vertically between the integrated heater and theintegrated silicon temperature sensor, the first material exhibiting alower temperature coefficient of resonant frequency (TCF) than silicon.2. The integrated wavelength selector of claim 1, wherein the integratedheater is a ring substantially overlying the ring waveguide of the firstmaterial.
 3. The integrated wavelength selector of claim 1, wherein thefirst material comprises silicon nitride.
 4. The integrated wavelengthselector of claim 1, wherein the integrated silicon temperature sensoris a p-n diode ring.
 5. The integrated wavelength selector of claim 1,wherein the integrated wavelength selector is disposed in a cavity of atunable laser.
 6. The integrated wavelength selector of claim 5, furthercomprising a plurality of integrated heaters, integrated silicontemperature sensors, and ring waveguides disposed in the cavity of thetunable laser.
 7. The integrated wavelength selector of claim 1, whereinthe integrated wavelength selector is coupled to an output of a laserand configured as a wavelength locker.
 8. The integrated wavelengthselector of claim 1, further comprising a plurality of temperatureisolation trenches disposed inside and outside the integrated silicontemperature sensor.
 9. The integrated wavelength selector of claim 1,wherein the first material is silicon nitride, and wherein the ringwaveguide has a larger radius than the integrated silicon temperaturesensor.
 10. The integrated wavelength selector of claim 1, wherein thefirst material is silicon nitride, and wherein the integrated wavelengthselector further comprises first and second waveguides disposed onopposite sides of the ring waveguide.
 11. A tunable wavelength selector,comprising: a micro-heater; a micro-temperature sensor formed ofsilicon; and an optical filter of a first material disposed verticallybetween the micro-heater and the micro-temperature sensor, the firstmaterial exhibiting a lower temperature coefficient of resonantfrequency (TCF) than silicon.
 12. The tunable wavelength selector ofclaim 11, wherein the micro-heater is ring-shaped and configured tosubstantially overlie the optical filter.
 13. The tunable wavelengthselector of claim 11, wherein the first material comprises siliconnitride.
 14. The tunable wavelength selector of claim 11, wherein themicro-temperature sensor is a p-n diode ring.
 15. An apparatus,comprising a tunable laser having a laser cavity, and the tunablewavelength selector of claim 11 disposed within the laser cavity. 16.The apparatus of claim 15, comprising a plurality of ring resonatorsdisposed within the laser cavity.
 17. An apparatus, comprising a tunablelaser and the tunable wavelength selector of claim 11 coupled to anoutput of the tunable laser.
 18. The tunable wavelength selector ofclaim 11, further comprising a plurality of temperature isolationtrenches disposed inside and outside the micro-temperature sensor. 19.The tunable wavelength selector of claim 11, wherein the first materialis silicon nitride, the optical filter is a ring resonator, and whereinthe ring resonator has a larger radius than the micro-temperaturesensor.
 20. The tunable wavelength selector of claim 11, wherein thefirst material is silicon nitride, and wherein the optical filtercomprises first and second waveguides disposed on opposite sides of aring waveguide.